Ethylene interpolymers products are widely used in rotomolding applications to produce a wide variety of manufactured articles. Non-limiting examples of rotomolded articles include: toys, bins, containers, helmets, boats, large tanks. Such articles are produced using rotomolding equipment, non-limiting examples include: clamshell machines, shuttle machines, swing arm machines, carousel machines and the like. There is a need to improve the Environmental Stress Crack Resistance (ESCR) of rotomolding articles, while maintaining the stiffness and impact properties, for example ARM Impact at low temperatures (−40° C.). It is well known to those of ordinary experience in the art that the stiffness of a rotomolded article can be increased by increasing the density of the ethylene interpolymer; however, it is also well known that ESCR decreases as density increases.
High ESCR in a rotomolding resin is particularly important when making rotomolded containers that hold chemicals. Non-limiting examples of chemicals include detergents, glycols, sulfuric acid, sodium hydroxide, hydrochloric acid, acetic acid, oils and fuels. Non-limiting examples of a rotomolded container size include from a few liters up to 300-500 gallons. In many applications, rotomolded containers also have complex shapes and such shapes require a high flow resin. By high flow resin, we mean a polymer having a melt flow index 12 from about 4 to about 6 dg/min (at 190° C. and 2.16 kg load). However, with increasing melt index (or decreasing weight average molecular weight) ESCR is reduced. Thus, the following is a technical challenge: designing and manufacturing high flow resins that produce rotomolded parts having excellent ESCR properties. The rotomolding resin disclosed herein are well suited to address this technical challenge.
The ethylene interpolymer products (rotomolding resins) disclosed were produced in a solution polymerization process, where catalyst components, solvent, monomers and hydrogen are fed under pressure to more than one reactor. For ethylene homo polymerization, or ethylene copolymerization, solution reactor temperatures can range from about 80° C. to about 300° C. while pressures generally range from about 3 MPag to about 45 MPag and the ethylene interpolymer produced remains dissolved in the solvent. The residence time of the solvent in the reactor is relatively short, for example, from about 1 second to about 20 minutes. The solution process can be operated under a wide range of process conditions that allow the production of a wide variety of ethylene interpolymers. Post reactor, the polymerization reaction is quenched to prevent further polymerization, by adding a catalyst deactivator, and passivated, by adding an acid scavenger. Once passivated, the polymer solution is forwarded to a polymer recovery operation where the ethylene interpolymer product is separated from process solvent, unreacted residual ethylene and unreacted optional α-olefin(s).